Hammocks and stress on standing rigging

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rhumbunctious

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I see alot of hammocks strung up on decks, attached to the standing rigging (e.g. forestay and shrouds, etc.) and while it's clear that this works, I've been wondering what kind of stress and possible reduction in lifespan this may cause to the rigging. Since the stresses on standing rigging are expected to be vertical along the length of the wire, and the hammock places a bending stress horizontally, what is that doing to the wire? Is it causing or increasing work hardening at the point of stress/bend?

I'm wanting to string a hammock up myself, but am concerned about the long term affects/damage to the rigging.

Any experts care to comment?
 
Except on the very smallest boats, the loads a hammock will impose are a fraction of those exerted by a loaded rig under sail in a good breeze. I wouldn't anticipate any problems - the only exception would be to make sure any load imposed on the headfoil is not so concentrated as to actually deform the foil profile.
 
some basic figures ( I am not going to go into Newtons :) ) :-

Assuming a person and hammock of mass of 100kg....

If the angle between the two points of suspension is 45 degs, then the load in each 'leg' is 50 kg. (That is a pull of 50 kg at roughly 90 to 45 degs to the rigging depending on the angle of the rigging from the vertical.

If the angle is 120 degs (more likely) then there's a pull of 100kg in each leg.

If the angle is 150 degs (quite possible), then the pull on each leg is 200 kg.


Now, without knowing the rigging angle from the vertical, it is not possible to calculate the increased load on the ends of the wire, but when used as a hammock support, rigging is not going to deviate more than a few degs from the unloaded (non-hammock) line, so the likely load force of between 100 and 200 kg will be 4, 5, 6, x times greater.
 
sarabande said:
some basic figures ( I am not going to go into Newtons :) ) :-

Assuming a person and hammock of mass of 100kg....

If the angle between the two points of suspension is 45 degs, then the load in each 'leg' is 50 kg. (That is a pull of 50 kg at roughly 90 to 45 degs to the rigging depending on the angle of the rigging from the vertical.

If the angle is 120 degs (more likely) then there's a pull of 100kg in each leg.

If the angle is 150 degs (quite possible), then the pull on each leg is 200 kg.


Now, without knowing the rigging angle from the vertical, it is not possible to calculate the increased load on the ends of the wire, but when used as a hammock support, rigging is not going to deviate more than a few degs from the unloaded (non-hammock) line, so the likely load force of between 100 and 200 kg will be 4, 5, 6, x times greater.
Click to expand...

Think your figures might be a bit out there...

i make it, assuming 100kg fat bloke..

angle betweeen the 2 bits of string at each end of the hammock;

90deg - horizontal 50kg, down the string 70Kg
120deg - horizontal 60kg, down the leg 78Kg
150deg - Horizontal 86Kg, down the leg 100Kg

Or little more if he's swinging ;)
 
you had better argue with My Lords Commissioners of the Admiralty :).

I learned those figures in the 60's as a cadet, and checked them from the Manual of Seamanship before the OP.


Always willing to see an explanation, though.
 
sarabande said:
you had better argue with My Lords Commissioners of the Admiralty :).

I learned those figures in the 60's as a cadet, and checked them from the Manual of Seamanship before the OP.


Always willing to see an explanation, though.
Click to expand...

I am completely and utterly wrong and you are completely and utterly right :) :)


Vertical load on hammock=100Kg
vertical load down each leg=50kg

if angle between vertical and leg is 45deg then..

force down leg = vertical load / cos(45) = 70

120 deg = 60 deg between vertical and leg = 50/cos(60)=100

150deg = 75deg = 193Kg

I drew it up in cad initially but got the angles wrong :(
 
easy enough; done it myself many times. ;)

What is fascinating is the strains induced in a column or tight wire when a side load is applied. The video of a crewman falling on the mast of an AC45, and breaking it is an example.

It is also what makes 'sweating' a mooring rope such a powerful tool when the rope is nearly tight. A tight mooring line multiplies many times the side force put into it.
 
Similalry I had wondered about the common practice of using the shrouds to heave oneself aboard over the guard rail etc. If you take the calculation to infinite conclusion, ie you have a wire so taut that it doesnt give at all then the smallest sideways force creates an infinite force along the wire!
 
Been using a hammock from mast to forestay for years . Rigging and mast still standing so just do it. Enjoy and have a Pimms!!
 
Look at it this way.
You are closed hauled in a F7.

Does the mast fall down? No.
You apply 100kg of force side ways on the weather shroud.
Is the leward shroud now tight or still slack?

I wouldn't mind betting that it is still slack.
So the force on the weather shroud has not changed significantly. All you have done is prevent the mast leaning quite so far over to leward by the amount you have effectively shortened the length of the windward shroud.

So - just go ahead an put up the hammock.
No way is the mast coming down.
 
I always hung the hammock under the spinny pole, that takes all the strain off the rig, and allows it to be slung just inches off the deck, for ease of access - and your glass is to hand.
Remember to fill it with cushions as well, for that extra long comfortable session.
Sunburn can be a problem though...
 
Billjratt said:
I always hung the hammock under the spinny pole, that takes all the strain off the rig, and allows it to be slung just inches off the deck, for ease of access - and your glass is to hand.
Remember to fill it with cushions as well, for that extra long comfortable session.
Sunburn can be a problem though...
Click to expand...
Keep an eye on the crew though. They may swing you out over the rail and give you a push....
 
Billjratt said:
I always hung the hammock under the spinny pole, that takes all the strain off the rig, and allows it to be slung just inches off the deck, for ease of access - and your glass is to hand.
Remember to fill it with cushions as well, for that extra long comfortable session.
Sunburn can be a problem though...
Click to expand...

Thanks for the spinny pole idea. Perfect.
 
A word of warning, I was met my an unexepected situation this summer when I was entering an anchorage in a flat sea with a member of crew asleep in a hammock. Just as I approaching, a very large super yacht was leaving, it immeadiately powered up and by the time it passsed 100m in front of me it must have been doing 25knts. The wake was huge, so the natural thing is to head staight into it. The first wave did little to the hammock but the second and third wave sent the chap in the hammock 6ft into the air, the look of shear terror on his face will live with me for a long time. Luckily no damage to the standing rigging, but the crew member didn't sleep in the hammock for the rest of the trip.
 
skyflyer said:
Similalry I had wondered about the common practice of using the shrouds to heave oneself aboard over the guard rail etc. If you take the calculation to infinite conclusion, ie you have a wire so taut that it doesnt give at all then the smallest sideways force creates an infinite force along the wire!
Click to expand...

But better than putting the strain on the stantions....
 
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